PhD, professor of psychologyand neuroscience at theUniversity of Michigan.

“One important target for
therapeutic intervention
may be the development of
effective strategies to cope
with the insidious effects of
reward-associated stimuli.”

Stuck on signals

Years ago, when Rutgers
University psychologist Arthur
Tomie, PhD, was studying
classical conditioning in his
lab rats, he noticed something
strange. Just as in Ivan
Pavlov’s famous conditioning
experiments, Tomie’s model
involved a first stimulus that
signaled the occurrence of
a second stimulus — in this
case, a metal lever dropped

“One hypothesis isthat, for whateverreason, these animalshave different brains.

In sign-trackers,you have thisdopamine-dependantmotivational process.

In the goal-trackers,or TV commercial) oftenhave the power to temptpeople even when theyaren’t hungry.

Studies have shown
that obese people are more
attracted to food signals
than people of healthy
weights, Robinson notes.

And drug addicts are morelikely than non-addicts tobe drawn to signals theyassociate with their habit.

“A lot of behavior, be it
human or animal, is really
controlled by cues in the
environment that predict
rewards,” he says.

Yet it’s not clear whysome people might bemore focused on cuesthan others, nor how thatdifference might contributeinto the rats’ cages, signalingthat a food treat was about to

something else is goingto addictions or overeating.

That’s where the rodentsappear. The animals quicklycan help, Robinson believes.

learned to connect the signalwith the reward, and when the

on psychologically.”Over the last several years,he has used a rat modelmetal lever appeared, many ofsimilar to Tomie’s to explorethe rats darted to the corner ofthe cage where food would bedelivered.

TERRY ROBINSONUniversity of Michiganwhy some animals are soeasily tempted by signals intheir environments.

Some of the rats, however,were drawn to the lever itself.

Rather than run to the fooddrop, the rats licked and gnawed the inedible metal lever asthough it were rat kibble (Clinical Psychology Reviews, 1995).

This action wasn’t totally unfamiliar, says Tomie. Researchers
had noted similar behavior in pigeons back in the 1960s
(Journal of the Experimental Analysis of Behavior, 1968). But
watching the rats, Tomie was reminded of drug addiction in
people. “What intrigued me is that the animals didn’t appear
to be able to control themselves,” Tomie says. He even tried
withholding the food reward from those rats that contacted
the lever first. Still, a handful of rats couldn’t resist the lever’s
lure. “The animals are unable to stop themselves, to their
detriment,” he says.

Robinson has identifiedtwo distinct behaviors among the rodents. For some rats (the“goal-trackers”) the final reward is most important. They runfor the food cup as soon as they spot the signaling lever. Forothers (the “sign-trackers”) the cue itself holds the most sway.

These are the animals that Tomie saw licking and gnawing onthe lever.

The sign trackers seem to be particularly prone to addictionor other maladaptive behaviors, Robinson says. Rodents thatkeep their eyes on the lever are much more likely to seek drugsor swallow alcohol from a sipper.

A compulsion for cues seems to extend across many typesof behaviors. Sign-tracking rats have trouble resisting cues ingeneral, whether they’re associated with food or with drugs,desperately want to quit drug or alcohol consumption butexperience a triggered relapse, Tomie says. For an alcoholic,for example, the trigger may be the sight of a cocktail glassor the flickering neon sign in front of a favored wateringhole. Similarly, food-related signals (such as a fast-food signsuch as cocaine. In fact, by identifying sign-trackers using thefood-pellet model, Robinson can take rats that have never beenexposed to drugs and correctly predict which animals are likelyto exhibit drug-seeking behaviors when introduced to cocaine(Biological Psychiatry, 2010).